1. Field of the Invention
The present invention concerns a gradient coil and a method for manufacturing a gradient coil.
2. Description of the Prior Art
A typical magnetic resonance apparatus has a gradient coil system that contains three (partial) gradient coils. For example, a magnetic field gradient in the X-direction is generated with the aid of a first gradient coil while a magnetic field gradient in the Y-direction is generated with the aid of a second gradient coil. Finally, a third gradient coil generates a magnetic field gradient in the Z-direction.
The XY gradient coils are known as “saddle coils” due to their shaped design.
It is known to design saddle coils with the use of bundled individual wires. For example, FIG. 3 shows such a saddle coil according to the prior art, which here is designed as part of a Y-gradient coil.
In this coil design, conductor loops that typically are formed of one to six bundled individual wires are fixed on a support plate—for example by adhesion. The individual wires of a conductor loop are insulated from one another with an enamel insulation layer, typically of 2×10 μm thickness. A common current signal flows through the individual wires of the conductor loop in order to form the Y-gradient field, or the Y-magnetic field, in an examination region of the magnetic resonance system.
Such saddle coils enable an optimized current density to be achieved in a predetermined central coil region in order to form the desired magnetic field in the examination region.
It is also known to fashion saddle coils using an electrically conductive plate. For example, elliptically running divider structures are milled as what are known as traces into the electrically conductive plate. The plate is subsequently brought into the saddle shape, for example by curving the plate in the form of a half-cylinder shell.
The traces can also be produced by cutting methods (water jet cutting methods or laser cutting methods, or by punching techniques, etc.
Conductor structures that, charged with a current signal, form a desired X-gradient field or Y-gradient field are formed via the curvature of the plate in the saddle shape and via the dividing traces.
The magnetic field efficiency is determined by a maximum achievable current density in a middle region of the plate. The requirement for a minimal insulation distance between the coil windings or, respectively, conductor structures therefore results in this middle region. Minimal conductor structure cross-sections are thereby necessary.
The power consumption of a gradient system from the mains network is determined by the ohmic resistance of the gradient coils. A requirement for a maximum cross-section of the respective conductor structures results from this in order to be able to use a mains power that is available only to a limited degree to the customer.
Given currents of 500 A to 1000 A that are typical today, in general 20 to 30 individual conductor loops result that are to be realized on the plate of the later saddle coil.
The advantage of a saddle coil produced from an electrically conductive plate is that the gradient coil possesses a very small resistance because a large-area conductive surface that is merely reduced by the width of the trace is available as a conductor structure. Depending on the technology, this trace can be very narrow—even only a few millimeters wide.
On both of the illustrated saddle coil designs, a distance (designated as a gap measurement) between the individual coil windings of the saddle coil is limiting.
Given use of an electrically conductive plate in the saddle coil design, the plate and its traces are compressed and expanded in the formation of the saddle shape. This shaping has a direct influence on the ratio of the trace width to the plate thickness. The thicker the plate, the larger a trace width that must be selected in order to prevent short circuits between the edges bordering the trace upon shaping.
In gradient coils known as microscopy gradient coils (with typical inner diameters of less than 100 mm), conductor structures are stamped in support structures that are as thin-walled as possible. Here copper cylinders with a wall thickness of w<=1 mm and a laser-cut gap measurement s of 0.2 mm are typical. Here the problem of the compression and expansion of a saddle coil does not occur.
In contrast to this, in whole-body gradient coils (with typical inner diameters of greater than 600 mm) the gradient axes are produced from multiple partial coils. Due to the effort, here methods have been achieved that assume a flat plate.